Due to the increase of the demand for a high capacity data service, such as a variety of multimedia Internet services, as well as voice calls in a wireless communication market, a variety of wireless transmission technologies have been developed in order to satisfy the demand.
Active research for MIMO technology is in progress lately as a highly attractive field. Since the MIMO system uses multiple antennas at its transmission and reception port, respectively, it may increase a channel transmitting capacity in proportion to the number of antennas without an additional allocation of transmission power or frequency, in comparison with a system using a single antenna.
The MIMO technology can be classified into two techniques including CL-MIMO (Closed-Loop MIMO) and OL-MIMO (Open-Loop MIMO) according to whether a base station uses feedback information of a user in order to obtain information of a downlink (DL) channel. A SU-MIMO (Single-Use MIMO) system, which can transmit data to only one user at a time in the same frequency band, can use both the OL-MIMO scheme and the CL-MIMO scheme, but a MU-MIMO (Multiple-User MIMO) system, which can transmit data to several users at the same time in the same frequency band, can use only the CL-MIMO scheme. With the development of communication systems are developed, an active research for the MU-MIMO system capable of maximizing a system capacity is in progress, and thus the importance of the CL-MIMO scheme has increased.
In order to use the CL-MIMO scheme, in general, a base station recognizes DL channel information of users. The channel information includes Channel Direction Information (CDI) and Channel Quality Information (CQI). The CQI is normally reported from users. A method by which the base station obtains the DL-CDI of users is as follows.
The first method is to use a predetermined codebook, in which a user selects a codeword most accurately representing his/her DL-CDI from the predetermined codebook, and gives a feedback of a corresponding codeword index to the base station. The method of using the codebook has an advantage of reducing a feedback overhead, but has a disadvantage in that there is a limitation in the transmission of the various and accurate DL-CDI due to the use of the finite number of quantized codewords.
The second method is to receive a predetermined sounding signal from a user, in which the base station receives the sounding signal transmitted from the user, estimates UL (Uplink)-CDI, and then uses the UL-CDI as the DL-CDI based on the reciprocity of a channel. The method of using the sounding signal has disadvantages in that the method can be used only in a Time Division Duplex (TDD) system which divides the DL and the UL by a time domain. When several users transmit the sounding signal in the same frequency band at the same time, interference is caused by the sounding signal transmitted from users of other cells so that a quality of the sounding signal received by the base station is degraded. However, the method of using the sounding signal has an advantage in that it is possible to transmit more various and accurate DL-CDI, in comparison with the method of using the codebook.
When all the users transmit the sounding signal to the base station through the entire frequency band by using a part of UL resources in every frame for a DL scheduling of the base station, electric power allocated to the sounding signal for each frequency band decreases, and a quality of the sounding signal received in the base station is degraded due to interference caused by the sounding signal transmitted from users of other cells. As a result, a further problem occurs in that the accuracy of a beam forming matrix is deteriorated.